Matching tartan flashes?

[Jock Scot]

I personally like flashes, both of the plain colours and matching tartan variety. They can be used to match or contrast with your kilt, or jacket, or shirt, or tie, or pocket handkerchief. While Steve's assessment of their origin is undoubtedly correct, you can be sure that your tartan flashes will not unduly clash. I think it quite probable that scraps of tartan, likely matching one's kilt were regularly used to hold one's socks up.
If you like 'em, wear 'em. The keepers of the THCD rule book here have no police. Develop your own style, and that can include defying the XMarks THCD rule book.

Slainte

There is no rule book with THCD that I am aware of. There are things to consider for sure, but the whole essence of THCD is individual choice within a fairly deep knowledge of Highland attire matched to the occasion it is worn.

Just saying.